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United States Patent |
5,296,834
|
Urban
|
March 22, 1994
|
Thermostatic switch and bimetallic disc assembly therefor
Abstract
A bimetallic disc assembly for a bimetallic disc-actuated thermostatic
switch includes a disc retainer, a bimetallic disc received in assembled
position on the disc retainer and a metallic retainer cap for retaining
the disc in assembled position on the disc retainer and for securing the
assembly to the body portion of the thermostatic switch. The disc retainer
is adapted to minimize heat flow from the metallic cap to the disc
retainer by minimizing the areas of contact between the disc retainer and
the cap. The disc retainer is integrally formed from a ceramic material
and it includes a central hub having an axial bore extending therethrough,
a circular flange extending radially outwardly from the hub, a circular
shoulder extending upwardly from the flange, and a peripheral rim
extending upwardly from the shoulder. The peripheral rim includes a
horizontal edge with a plurality of circumferentially spaced projections
or bumps which extend upwardly therefrom. The metal cap is received on the
disc retainer and rests upon the projections or bumps which effectively
reduce the contact area between the cap and the retainer and thereby
minimize the transfer of heat from the cap to the disc retainer. By
reducing the heat transfer from the cap to the retainer, heat is more
easily transferred from the metallic cap to the bimetallic disc, thereby
improving response time.
Inventors:
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Urban; Anthony F. (Warren, RI)
|
Assignee:
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Ellmwood Sensors, Inc. (Pawtucket, RI)
|
Appl. No.:
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074932 |
Filed:
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June 10, 1993 |
Current U.S. Class: |
337/372; 337/354; 337/380 |
Intern'l Class: |
H01H 037/04; H01H 037/52 |
Field of Search: |
337/372,380,112,113,381,365,348,354
|
References Cited
U.S. Patent Documents
4754252 | Jun., 1988 | Craig, III | 337/380.
|
4952901 | Aug., 1990 | Chrupcala et al. | 337/372.
|
Primary Examiner: Broome; Harold
Attorney, Agent or Firm: Salter, Michaelson & Benson
Claims
What is claimed is:
1. A disc retainer for a thermostatic switch of the type actuated by a
bimetallic disc comprising:
a circular flange portion having an axial bore extending therethrough;
a circular shoulder portion extending upwardly from a first side of said
flange portion, said shoulder portion engaging a peripheral edge portion
of said bimetallic disc and supporting said disc above said first side of
said flange; and
a peripheral rim portion extending upwardly from said shoulder portion,
said peripheral rim portion having a terminal edge, and a plurality of
circumferentially spaced projections which extend upwardly from said
terminal edge.
2. In the disc retainer of claim 1, said peripheral rim portion having
eight circumferentially spaced projections.
3. The disc retainer of claim 1 comprising an integrally molded ceramic
material.
4. A bimetallic disc assembly for a bimetallic disc-actuated thermostatic
switch comprising:
a disc retainer comprising a circular flange portion having an axial bore
extending therethrough, a circular shoulder portion extending upwardly
from a first side of said flange portion, and a peripheral rim portion
extending upwardly from said shoulder portion, said peripheral rim portion
having a terminal edge and a plurality of circumferentially spaced
projections which extend upwardly from said terminal edge;
a bimetallic disc received in assembled position on said disc retainer
wherein a peripheral edge of said disc is supported on said shoulder
portion; and
a metallic cap received on said disc retainer for retaining said disc in
assembled position and for securing said disc assembly to a body portion
of said thermostatic switch, said retainer cap including an end wall
portion having an enlarged central opening therein and a skirt portion
deepening from said end wall portion and encircling said disc retainer,
said end wall resting on said projections and retaining said disc in
assembled position, said skirt portion being receivable on said body
portion for securing said assembly thereto.
5. In the bimetallic disc assembly of claim 4, said disc retainer being
integrally molded from a ceramic material.
6. In the bimetallic disc assembly of claim 4, said peripheral rim portion
having typically eight circumferentially spaced projections.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The instant invention relates to bimetallic disc-actuated thermostatic
switches and more particularly to a bimetallic disc assembly therefor
which is adapted for minimizing the response time of the switch.
It is known that many household, office and industrial machines and
appliances now include highly sensitive electrical components that may be
damaged by thermal overload conditions. In this regard, thermostatic
switches have heretofore been known as high limit devices for preventing
thermal overload damage to machines or appliances containing such
sensitive electrical components. In this regard, thermostatic switches are
operative for interrupting the electrical connections of the main heat
generating components of the machine or appliance in response to thermal
overload conditions. However, since it has been found that thermal
overload conditions can develop very rapidly, it is essential for the
thermostatic switches which are used in these devices to have a rapid
response time in order to avoid damage to the machine components.
The U.S. patents to Craig, III U.S. Pat. No. 4,754,252 and Chrupcala U.S.
Pat. No. 4,952,901, which represent the closest prior art to the subject
invention of which the applicant is aware, are specifically concerned with
minimizing response time in thermostatic switches. The Craig patent
concerns a disc retainer and cap assembly which cooperate to increase air
flow across the surface of the bimetallic disc. The cap has a plurality of
air circulation vents extending therethrough and the disc support or
retainer is designed so that exterior ambient air can pass through the air
circulation openings and across the underside of the bimetallic disc to
provide increased responsiveness to ambient conditions. While the Craig
device is effective for reducing response time, it also presents several
design problems which make it difficult to assemble. In particular, the
radial disc supports must be radially aligned with the areas between the
vents so that the disc supports do not block air flow through the vents.
It can be appreciated that this rotational alignment is very difficult to
maintain while the cap is positioned on the switch body and crimped in
place.
The patent to Chrupcala discloses a disc retainer and aluminum cap assembly
which is adapted to minimize the conduction of heat from the bimetallic
disc to the disc retainer. The disc retainer includes a central hub
portion, a plurality of fingers which radiate outwardly from the hub
portion, a plurality of disc retainer projections on the fingers and a
plurality of disc support shoulders on the fingers adjacent the disc
retainer projections. The fingers have open notches therebetween for
reducing the cross-sectional area of the disc retainer which may conduct
heat from the bimetallic disc to the body portion of the thermostatic
switch. The aluminum cap includes an end wall portion having an enlarged
opening therein and a skirt portion having a plurality of apertures or
slots therein which reduce the cross-sectional area of the skirt for
conducting heat from the bimetallic disc to the body portion of the
switch. The Chrupcala device also presents several problems. More
specifically, the material from which the disc retainer can be formed is
limited to plastic thermosetting materials which are easy to mold and
resistant to breakage during manufacturing and handling. The problem here
is that thermosetting materials have a relatively low melting point which
limits use of the switch to applications where temperature does not exceed
375-400 degrees F. In addition, it has been found that it is difficult to
crimp the aluminum perforated cap, and it has further been found that the
crimping of the cap tends to deform or warp the cap, which may result in
pressure on the disc which in turn may adversely affect the snap-action
thereof. It is pointed out that these types of disc retainers cannot be
formed from a ceramic material because the fingers would be very brittle
and would very easily break during the manufacturing process.
The instant invention provides an improved bimetallic disc assembly for a
bimetallic disc-actuated thermostatic switch. In this regard, the disc
assembly includes an integrally molded ceramic disc retainer, a bimetallic
disc which is received on the disc retainer, and a metallic cap which is
received over the disc retainer. The disc retainer is adapted to minimize
heat flow from the metallic cap to the disc retainer by minimizing the
areas of contact between the disc retainer and the cap. The ceramic
construction of the instant disc retainer also allows it to be used in a
broader temperature range than the prior art plastic disc retainers. In
particular, the instant disc retainer comprises a central hub having an
axial bore extending therethrough, a circular flange extending radially
outwardly from the central hub, a circular support shoulder extending
upwardly from the flange, and a peripheral rim extending upwardly from the
support shoulder. The peripheral rim has a plurality of circumferentially
spaced projections, or bumps which extend upwardly therefrom. The metal
cap is received on the disc retainer so that it rests on top of the
projections or bumps on the peripheral rim. The bumps effectively minimize
the contact area between the cap and the disc retainer and thereby reduce
the transfer of heat from the cap to the disc retainer. By reducing the
transfer of heat from the cap to the disc retainer, heat is more quickly
transferred from the cap to the bimetallic disc, thereby improving
response time.
Accordingly, it is an object of the instant invention to provide a
thermostatic switch which has a quick response time to changes in wide
ranges of ambient temperature conditions.
It is another object to provide a disc retainer for a thermostatic switch
which may be made of ceramic and which is adapted to increase response
time to changes in ambient temperature conditions.
It is still another object to provide a disc retainer which minimizes
contact area between the cap and disc retainer.
It is still another object to provide a thermostatic switch which can be
used in a broad range of temperatures.
Other objects, features and advantages of the invention shall become
apparent as the description thereof proceeds when considered in connection
with the accompanying illustrative drawings.
DESCRIPTION OF THE DRAWINGS
In the drawings which illustrate the best mode presently contemplated for
carrying out the present invention:
FIG. 1 is a perspective view of the thermostatic switch of the instant
invention;
FIG. 2 is an exploded perspective view showing the components of this
switch;
FIG. 3 is a side elevational view of the switch without the cap, partially
in cross-section and with portions broken away for purposes of
illustration; and
FIG. 4 is a similar view with the cap mounted on the disc retainer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, a bimetallic disc-actuated thermostatic
switch which includes the bimetallic disc assembly of the instant
invention is illustrated and generally indicated at 10 in FIGS. 1 through
4. The thermostatic switch 10 includes a bimetallic disc assembly
generally indicated at 12, a switching assembly generally indicated at 14,
including first and second external electrical terminals 16 and 18,
respectively, and a switch body generally indicated at 20. The disc
assembly includes a disc retainer generally indicated at 22, a bimetallic
disc 24, and a metallic cap generally indicated at 26. The disc retainer
22 and switching assembly 14 are assembled with the switch body 20 so that
the bimetallic disc 24 is operative for actuating the switching assembly
14 for selectively effecting and interrupting electrical continuity
between the electrical terminals, 16 and 18, in response to a
predetermined temperature condition.
The disc retainer 22 is preferably integrally molded from a suitable
ceramic material and it includes a central hub portion 28 having an axial
bore 30 extending therethrough, a circular flange portion 32 extending
radially outwardly from the hub portion 28, a circular shoulder portion 34
extending upwardly from the peripheral edge of the flange 32, and a
peripheral rim portion 36 extending upwardly from the shoulder portion 34.
The peripheral rim portion 36 includes a horizontal edge 38 and a
plurality of projections or bumps 40 on the horizontal edge 38. The
ceramic construction of the instant disc retainer 22 provides a distinct
advantage over the prior art plastic disc retainers in that it is not
limited by temperature considerations which must be taken into account
when using thermosetting plastic disc retainers. The ceramic construction
of the disc retainer 22 thus permits the instant thermostatic switch 10 to
be used in a broad band of temperatures which range from cryogenic to
greater than 600 degrees F.
The bimetallic disc 24 preferably comprises a conventional bimetallic disc
which is flex-responsive to a predetermined temperature condition as a
result of the differences in thermal expansion properties of the metals
from which it is formed. The disc 24 is dimensioned so that a peripheral
edge 42 is received and supported on the shoulder portion 34 of the disc
retainer 22.
The retainer cap 26 is preferably made from a suitable sheet metal and it
includes a substantially flat end wall portion 44 having an enlarged
central opening 46 therein, and a skirt or sidewall portion 48 which
deepens from the end wall portion 44. The cap 26 is dimensioned to be
received on the disc retainer 22 for retaining the bimetallic disc 24 in
assembled position on the disc retainer 22. More specifically, the cap 26
is received on the horizontal edge portion 38 of the disc retainer 22 so
that it rests on top of the projections or bumps 40 thereon. It has been
found that the metal cap 26 has a large surface area which is directly
exposed to ambient air and that it is therefore extremely effective in
accumulating heat from the ambient air. The bumps 40 on the disc retainer
22 effectively minimize the contact area between the metal cap 26 and the
ceramic retainer 22 and thereby minimize the transfer of heat from the cap
26 to the disc retainer 22. By reducing the transfer of heat from the cap
26 to the disc retainer 22, the heat accumulated in the cap 26 is more
easily transferred from the cap 26 to the bimetallic disc 24, thereby
improving response time.
The body portion 20 of the switch 10 is generally of conventional
construction and it is preferably manufactured from a suitable high
temperature insulating material such as a phenolic or ceramic, as
generally used on thermostatic controls. The switch body 20 includes a
bottom wall portion 50, an upstanding sidewall portion 52, and an enlarged
upper lip 54 on the terminal edge of the sidewall portion 52. An inner
shoulder 56 having an aperture 58 therethrough is formed in the interior
of the body portion 20 and a recess 60 is formed on the outer side of the
bottom wall 50.
The switching assembly 14 is received and assembled in the body portion 20
and it includes a fixed contact 62 which is secured to the inner shoulder
56 with a rivet 64 which passes through opening 65 in the fixed contact
62. The rivet 64 is received in an eyelet 66 which is in turn received in
the aperture 58. The first terminal 16 is received in the recess 60 on the
outer side of the bottom wall 50 and the rivet 64 extends through aperture
17 in the first terminal 16 for securing the first terminal 16 to the body
portion 20 so that it is electrically connected to the fixed contact 62
through the eyelet 66 and the rivet 64. The switching assembly 14 further
comprises a moveable contact assembly generally indicated at 68 including
a resilient contact arm 70 having a moveable contact element 72 thereon, a
rivet 74 and an eyelet 76. The rivet 74 extends through aperture 75 in the
base portion of the moveable contact assembly 68 and it is received in the
eyelet 76 which is in turn received in an aperture (not shown) in the
bottom wall 50 of the body portion 20. The second terminal 18 is assembled
in the recess 60 in the bottom wall 50 so that it is spaced from the first
terminal 16 and the rivet 74 passes through aperture 19 in the second
terminal 18 for securing it to the body portion 20. When the second
terminal 18 and the moveable contact assembly 68 are secured to the body
portion 20 in this manner, the rivet 74 and eyelet 76 effectively
establish electrical communication between the moveable contact assembly
68 and the second terminal 18. When the moveable contact assembly 68 is
assembled in the body portion 20 in this manner, the arm 70 is normally
positioned so that the contact element 72 engages the fixed contact 62 to
establish electrical continuity between the first and second terminals, 16
and 18 respectively. The resilient arm 70 is deflectable downwardly to
separate the moveable contact element 72 from the fixed contact element 62
in order to interrupt electrical continuity between the first and second
terminals, 16 and 18 respectively. Specifically, a transfer pin 78 extends
through aperture 30 into engagement with disc 24 whereby flexing of the
latter in response to a predetermined temperature condition causes the
transfer pin to move the moveable contact arm 70 downwardly to break
electrical continuity.
The disc assembly 12 is assembled on the body portion 20 so that a
peripheral edge of the flange portion 32 of the disc retainer 22 rests on
the upper edge of the lip 54 and the lower edge portion of the skirt
portion 48 of the cap 26 is crimped inwardly around the lower edge of the
lip 54 to permanently secure the disc assembly 12 to the body portion 20.
As previously stated, the disc assembly 12 is assembled with the body
portion 20 so that the transfer pin 78 is received in the axial bore 30 in
the disc retainer 22, and accordingly, the transfer pin 78 extends between
the bimetallic disc 24 and the resilient contact arm 70. Further, the
transfer pin 78 is dimensioned so that when the bimetallic disc 24 is in
an "upwardly" flexed disposition, the arm 70 is in an "at rest" position
wherein the moveable contact element 72 engages the fixed contact 62.
However, when the disc 24 snaps or flexes to a "downwardly" flexed
position in response to a predetermined temperature condition, the
transfer pin 78 moves the arm 70 downwardly to move the moveable contact
element 72 to a position of spaced disengagement from the fixed contact
62. Accordingly, electrical continuity between the first and second
terminals, 16 and 18 respectively, can be established or interrupted in a
conventional manner in response to a predetermined temperature condition.
It is seen therefore that the instant invention provides an effective
bimetallic disc assembly for a bimetallic disc-actuated thermostatic
switch. The disc retainer is provided with a plurality of bumps or
projections which minimize the contact area between the metal cap and the
disc retainer thereby reducing the transfer of heat from the cap to the
disc retainer. By reducing the transfer of heat from the cap to the disc
retainer, ambient heat accumulated in the cap can more easily flow from
the cap to the bimetallic disc thereby making the disc more responsive to
changes in ambient temperature and improving response time. In addition,
the construction of the disc retainer lends itself to being made of
ceramic, since there are no parts that are susceptible to breakage, and
since ceramic may be effectively used in high temperature applications,
this is a decided advantage. For these reasons the bimetallic disc
assembly and disc retainer of the instant invention are thought to
represent significant advancements in the art which have substantial
commercial merit.
While there is shown and described herein certain specific structure(s)
embodying the invention, it will be manifest to those skilled in the art
that various modifications and rearrangements of the parts may be made
without departing from the spirit and scope of the underlying inventive
concept and that the same is not limited to the particular forms herein
shown and described except insofar as indicated by the scope of the
appended claims.
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